Fouling or biofouling organisms attach to underwater structures on which they grow and develop. Growth of these organisms on surfaces associated with power plant cooling water systems can interfere with the efficient operation of the plant when the organisms block the flow of cooling water. The most common anti-fouling treatment used by power plants is water chlorination. However, this approach has adverse environmental effects on aquatic life, and recent publications have raised concerns about its long term human health effects.
Barnacles attach in large quantities to floating structures. When attached to the hull of a boat or ship, for example, barnacles increase drag and impair maneuverability. On buoys, their added weight lowers the floatation line, sometimes to the point of sinking the buoy. One method used to avoid barnacles is to paint the structure with an anti-fouling paint. However, these paints leak toxic chemicals into the water which are hazardous to life in the waterways. Furthermore, the efficacy of the paint decreases quickly with time so that the floating structures have to be hauled out for periodic scraping and painting with considerable cost and down-time.
Vibrations of certain frequencies can repel aquatic life forms and there is a need for a suitable anti-fouling system for small vessels applying this theory. One method is disclosed in U.S. Pat. No. 2,366,162 to Vang et al. which is directed to reducing the skin friction of water by vibration.
Great Britain Patent Nos. 703,158 and 719,650 refer to a method for minimizing marine growths on ship hulls by generating ultrasonic frequencies through piezoelectric transducers. The system requires a prime mover such as a steam turbine which moves an alternator to generate electrical power for the system. An oscillator is used to supply the ultrasonic frequencies.
An anti-fouling system based on an electromagnetic transducer was the subject of a patent application to Zarate and Verge, U.S. Ser. No. 07/795,494, filed on Nov. 21, 1991, which was later abandoned on Jun. 3, 1992. The anti-fouling system disclosed in the above U.S. application comprises an electronic system for preventing biofouling of boats and other water structures by producing sonic and ultra sonic vibrations. The system further included a microprocessor based controller with very low duty cycle for low current consumption, four ports for resonator connections, and an electromagnetic membrane transducer or resonator with a small gap and a ferrite core to provide a large vibrational force with low energy consumption.
The Zarate and Verge system has a current limit set by the controller electronics. Variations of the sound level from port to port are observed due to variability of the value of the electronic components of the controller circuit that limits the maximum current to each resonator. Moreover, the acoustic power level from each resonator decreases significantly when additional resonators are connected to the same port of the controller unit. A decrease in the sound level reduces the effectiveness of the device and will eventually eliminate anti-fouling action. A desirable system operates in such a way that additional resonators can be connected to the same port without changes in the sound level produced by each individual resonator. This allows the system to be used without component adjustments nor modifications, for small or large structures, just by changing the number of resonators connected to each port. Furthermore, there should the smallest possible variations in the sound level from port to port.
In an effort to achieve these goals, the original design was modified to increase the ohmic resistance of the resonators. Consequently, the current through each resonator is limited by the battery voltage and the resonator resistance in such a way that the controller current compliance is only reached in the case of a short circuit. A problem with this approach is that there is a significant decrease in the sonic output of the resonator for the same current due to the ohmic losses. Decreasing the gap to increase the magnetic field to compensate for this loss produces manufacturing tolerance problems and increases the sensitivity of the resonator to pressure and temperature changes.
One way to solve the problem of variations with multiple transducers would be to use permanent magnet type transducers. Transducers of the permanent magnet type have been used for a long time, for example, as sound speakers. This type of transducer would not be adequate for the desired application, however, due to large cost and size required for the same effect as electromagnetic transducers. Further, the vibrational force of an electromagnet can be larger than that of a permanent magnetic transducer for an equivalent magnetic field. The maximum magnetic field is the saturation flux density of the magnetic material from which the pole pieces are constructed. In contrast, with a permanent magnet transducer, the magnetic field is equivalent to the permanence of the permanent magnet. Since the saturation flux is approximately twice the permanence, a larger vibrational force is realized with the electromagnetic transducer of this invention. Therefore, it is desired to provide a solution using electromagnetic transducers. A novel solution to this problem is proposed.
The resonators of Zarate and Verge can operate only under near surface conditions. They are water resistant but not waterproof at one or two meters of depth as is required for devices installed in large ships that include ballast water. The main problem with working at water depths of more than one meter is the collapse of the resonator diaphragm. A similar problem arises with internal pressure changes produced by large temperature variations. As a result of the internal and external pressure changes, the resonator diaphragm is deflected. This occurrence changes the magnet gap and can collapse the diaphragm if the external pressure is large enough. If, instead, the internal pressure increases, the gap increases and the sound level is reduced.
In ships carrying ballast water, the resonators would be mounted in the inside of the ship under 90 cm of water. This produces a ten percent increase of the external pressure above the atmospheric value. The resonators are also subject to temperature variations that change the inside air volume proportionally. In air, changes as large as twenty percent over the design temperature of 300.degree. K. can be expected. For units under water, the temperature changes are smaller than 10 percent. A novel method to avoid these limitations is proposed.
In the Zarate and Verge system, wires and connectors are subject to electrolytic corrosion even with silicone coated heat-shrink tubing. This problem is exacerbated in underwater operation. We propose a method to minimize this problem.
This invention seeks to overcome drawbacks of known anti-fouling systems and to provide a transducer suitable for underwater vibrational anti-fouling. This invention further seeks to provide an efficient system and method for producing a large anti-fouling effect using minimal power and low current consumption.